1. Field of the Invention
This invention relates to a surface potentiometer for measuring the surface potential of a conductor or an insulator in a non-contact fashion.
2. Description of the Prior Art
As the simple measurement of a surface potential, there has hitherto been employed a method which comprises rotating a chopper blade biased to a predetermined potential between a surface to be measured and a measuring electrode and converting into AC signals the positive and negative potentials appearing at various timings on the surface to be measured, but the AC signal has made it impossible to distinguish between the positive and the negative polarities of the potential of the surface to be measured. Also, during the measurement of a surface potential, impedance conversion has been effected by utilizing the fact that the input impedance of a field effect transistor is great, but this has left the problem about how to bias the gate of the field effect transistor. As the bias means, it has generally been practised to connet the gate of a field effect transistor 106 to a specific potential within the operating range by a resistor 120, as shown in FIG. 18 of the accompanying drawings, but to take out the induced signal without attenuating the same by making the best use of the input impedance of 10.sup.9 -10.sup.5 ohms of the field transistor, a high resistance of the same degree as that is required and realization of such a resistor has led to a bulkiness and expensiveness of the device and has lacked in stability. Also, when the induced signal exceeds the break down voltage of the field effect transistor, it has been impossible to compensate for that.